To control nuclear reactors, so as to obtain variable power in operation, units are moved, generally in the vertical direction, which absorb the neutrons inside the reactor-core in which the fuel elements are found.
As the elements absorbing the neutrons are generally introduced through the top of the core, their insertion is increased by a downward movement and decreased by an upward movement. In the first instance, a lessening of reactor power is obtained, and in the other instance, an increase in this power is obtained.
In the event of emergency shutdown of the reactor necessitated by malfunction, it is necessary to put the absorbent elements in maximum insertion position in the reactor core very rapidly in order to stop the nuclear reaction.
To do this, the absorbent elements are generally allowed to fall into maximum insertion position in the reactor core under the action of their own weight.
In the case of pressurized water nuclear reactors, for example, the units absorbing the neutrons are constituted by very long tubes containing a material which is strongly absorbent of neutrons. These very long tubes are grouped in units termed control clusters, each of these control clusters or rods being associated with a fuel assembly. To control the reactor, the control rods are moved vertically, which produces a movement of the tubes containing the absorbent material between the fuel rods which are in vertical position in the core and grouped so as to form fuel assemblies.
Various mechanical or electro-mechanical apparatuses have already been proposed for carrying out controlled movement of the control rods in the vertical direction and to obtain immediate dropping of these control rods upon emergency shutdown.
These mechanisms are, however, complex and bulky, which is inconvenient because the whole mechanism has to be housed in a sealed casing in communication with the interior of the reactor vessel and disposed in the upper part of this vessel.
Thus, pawl apparatuses and rack-and-pinion apparatuses have been proposed which necessitate special machining of a shaft, termed control shaft, extending the absorbent element upwards, so as to obtain a series of equally spaced slots on this shaft or a rack.
There is also a known grab for handling fuel assemblies in nuclear reactors. Vertical movements in both directions for raising and inserting fuel assemblies are carried out by a synchronized movement of two cables oppositely wound on two pulleys fixed on one and the same shaft borne by the grab. Opening or closing of the claws of the grab so as to take up or release the fuel elements is carried out by a differential movement of the two cables which produces a rotation of the pulleys borne by the grab and the actuating of the pivoting fingers of the grab by means of a complex kinematic chain.
The differential movement of the cables for opening or closing the fingers of the grab is obtained by rotating the bevel wheel of a differential of the conventional type, with bevel pinions engaged with two differential pinions allowing arms to be set in rotation to effect differential movement of the cables.
With such an apparatus, therefore, it is necessary to provide a first means allowing a perfectly synchronous movement of the two cables to be achieved to lift or lower the fuel elements, and a second independent means for opening or closing the fingers.
An apparatus of this type has never been proposed for effecting movement of the control rods of a nuclear reactor and for dropping these control rods upon emergency shutdown.
In the case of mechanisms for control rods, the use of cable apparatuses is unsuitable. In practice, the cables require large radii of curvature for winding and very large drums, so that it is difficult to house such apparatuses in the casings provided for the mechanisms. Furthermore, use of drums causes a movement in the axis of the cable during winding or unwinding.
Lastly, the cables are not suitable for use in a medium such as the high-temperature and high-pressure water or the liquid sodium which most often constitute the exchanger fluid of the reactor.
However, there are known apparatuses which use chains providing vertical portions assuring lifting and chain wheels to assure movement and guiding of these chains, these apparatuses allowing control of the movement of the load and avoidance of any slipping in operation, and also being far smaller in size.
However, a lifting apparatus has never been devised which has chains which can be used to move the control rods of a nuclear reactor and for rapidly actuating the dropping of these rods into maximum insertion position in the reactor core.